Ammonium nitrogen removal from wastewaters can often be accomplished by biological nitrification. Nitrification is the oxidation of ammonium nitrogen to nitrate nitrogen by nitrifying autotrophic bacteria. The activated sludge process is utilized to stabilize wastewater biologically under aerobic conditions. Generally, activated sludge systems can be adapted to achieve nitrification of wastewaters amenable to biological treatment.
Despite widespread application of nitrification in activated sludge systems, the process is often hampered by uncertain reliability due in part to the particular requirements for growth of the nitrifying bacteria. These requirements include appropriate pH levels and adequate dissolved oxygen concentrations. In addition, the low growth rate of the nitrifying bacteria constrain process reliability perhaps more than any other factor. The low growth rate is further depressed by decreased temperatures as experienced seasonally in northern climates. High wastewater flow and high ammonium nitrogen loadings, even as experienced in normal diurnal variations, can cause a decrease of nitrification efficiency. It is an object of this invention to improve nitrification in activated sludge systems, particularly during periods of stressed operation caused by low wastewater temperatures, high hydraulic loadings or high ammonium nitrogen loadings.
Nitrifying activated sludge systems pose the problem of high capital cost as compared to conventional activated sludge plants because they generally require substantially more aeration volume. In combined carbon oxidation-nitrification systems longer solids retention times and hydraulic retention times must be maintained necessitating increased aeration volume. In two-stage systems having separate carbonaceous removal and nitrification, the requirements for aeration and clarifier volumes may be even greater. Another object of this invention is to reduce the aeration volume usually required for a nitrifying activated sludge system.